In a production process used for the manufacture of containers from a thermoplastic, blanks for containers are produced from severed parts of extruded long tubes consisting of an amorphous thermoplastic, these blanks normally being called preforms or pre-mouldings. At one end, the severed pieces are shaped in such a way that they form the further mouth part of the container, whilst they are closed at the opposite end.
The present invention eliminates certain disadvantages connected with the production process indicated above, in accordance with the known technology.
The invention is suitable especially for the manufacture of containers from a thermoplastic of the polyester or polyamide type. Examples of such materials are polyethylene terephthalate, polyhexamethylene-adipamide, polycaprolactam, polyhexamethylene-sebacamide, polyethylene 2,6- and 1,5-naphthalate, polytetramethylene 1,2-dihydroxybenzoate and copolymers of ethylene terephthalate, ethylene isophthalate and similar polymers. The description of the invention below relates mainly to polyethylene terephthalate, called PET in the further text, but the invention is not restricted only to the use of either this material or one of the other materials already mentioned; instead, it is also applicable to many other thermoplastics.
For a better understanding of the existing problem and of the invention, several characteristic properties of the polyester polyethylene terephthalate are described below. From the literature, for example Properties of Polymers, by D. W. van Krevelen, Elsevier Scientific Publishing Company, 1976, it is known that the properties of the material change when amorphous polyethylene terephthalate is oriented. Some of these changes are shown in the diagrams, FIGS. 14.3 and 14.4 on pages 317 and 319 in the book "Properties of Polymers". The symbols used in the discussion below correspond to the symbols in the said book.
PET, like many other thermoplastics, can be oriented by stretching the material. Normally this stretching takes place at a temperature above the glass transition temperature Tg of the material. The strength properties of the material are improved by orienting. The literature shows that, in the case of the thermoplastic PET, an increase in the stretching ratio .LAMBDA., that is to say the ratio of the length of the stretched material to the length of the unstretched material, also leads to an increase in the improvement of the material properties. When the stretching ratio .LAMBDA. is increased from about 2 to a little more than 3, particularly large changes in the material properties are obtained. The strength in the direction of orientation is here markedly improved, whilst at the same time the density .rho. and likewise the crystallinity Xc rises and the glass transition temperature Tg is raised. It can be seen from the diagram on page 317 that, after stretching, with .LAMBDA. assuming the value of 3.1, the material withstands a force per unit area, which corresponds to .sigma.=10, coupled with a very small elongation, whilst the elongation at .LAMBDA.=2.8 is substantially larger. In the further text, the term "step" is sometimes used to designate orienting which is obtained by stretching, or a reduction in thickness by about 3 times, and which leads to the marked improvements of the material properties, indicated above.
The diagrams referred to above show changes which are obtained on mono-axial orientation of the material. In biaxial orientation, similar effects are obtained in both directions of orientation. Orientation is carried out by successive stretchings.
Improved material properties, corresponding to those which are obtained by the "step" defined above, are also obtained if an amorphous material is stretched until it flows and, before flowing, the material is at a temperature which is below the glass transition temperature Tg. In a rod being drawn, a reduction of the diameter of about 3 times results in the flow zone. On drawing, the flow zone is continuously displaced into the amorphous material, whilst at the same time the material, which has already undergone the state of flowing, resists the tensile forces of the test rod without an additional permanent stretching.
Furthermore, it has been found, surprisingly, that an improvement of the material properties, obtained in the "step", is also achieved even if the material is caused to flow by means of pressure and the pressure-generating element at the same time reduces the thickness of the material to about 1/3 of the previous thickness (this applies to PET), the reduction in the thickness leading to an elongation of the section of material by a factor of about 3 in a direction which is at right angles to the direction in which the reduction in thickness takes place.
For bottles, defined external diameters of the mouth with the associated thread are standarized and, in the technology known at present when using the moulding process described as an introduction, this determines the greatest diameter which is permissible in the blow-moulded container body. The reasons for this are explained in more detail in the following text. In order to obtain an amorphous starting material for the pieces of tube, which are to be shaped into pre-mouldings, the material must be cooled rapidly to below the glass transition temperature Tg after extruding--in the case of extruded tubes from which the pieces of tube are severed. In the case of excessive wall thickness, the material does not possess adequate heat conductivity to enable the central sections of the wall to be cooled as rapidly as required, so that the material located in the center becomes crystalline and opaque. For this reason, viewed theoretically, the largest possible wall thickness of the extruded tubes is less than 9 mm In practice, however, wall thicknesses of less than 4 mm are used as a rule. In fact, in blow-moulding of a pre-moulding having wall material of excessive thickness, problems arise due to the cooling of the material during the actual blow-moulding step and before the material reaches the wall of the mould. The blow-moulded container is no longer clear as glass and, instead, contains opaque white sections. In blow-moulding, in order to obtain containers having the requisite resistance against stresses and penetration of the container wall, the wall thickness of the finished container must not fall below a defined value. Moreover, a reduction of the external diameter of the tube during the shaping of the mouth part of the pre-moulding is not possible in accordance with known technology. The result is that the desired mouth diameter of the blow-moulded container is decisive for the diameter of the pre-moulding and thus for the maximum diameter of the blow-moulded container body. If bottles of large capacity are required, these bottles are extended, according to known technology, in the axial direction after they have reached the maximum possible diameter. In addition to the disadvantage of a certain instability, the extension represents an unsatisfactory utilization of the quantity of material in the container body since the requisite quantity of material per unit volume of storage capacity is greater than would be necessary if both the diameter and the length of the container body were adapted to the actual volume required. Moreover, the unnecessarily large surface of the container leads to a corresponding increase in the overall penetration of carbon dioxide during the storage of beverages containing carbonic acid.
To utilize the material properties of the material in the best way, it is desirable that the diameter of those parts of the pre-moulding which, after the blow-moulding step, represent the actual container body, is given a value which has the result that the material in the blow-moulded container body assumes the desired orientation. In containers of PET it is desirable that the material, in conjunction with blow-moulding, is biaxially stretched in such a way that the product of the stretchings is about 9.
The above shows that, according to known technology, the quantity of material in the mouth part is not determined by the calculated stresses but by the maximum diameter of the container body. As a rule, this leads to a considerable excess of material in the mouth part. For example, in a PET bottle of 1 liter capacity, the mouth part can, according to known technology, contain up to 25-30% of the total quantity of material. Disregarding the unpleasant appearance of the oversizing of the mouth part, this fact also results in a waste of material, which is of importance in the mass production of articles.
In the technology applied at present, the mouth part and adjacent neck parts consist of unoriented material, that is to say amorphous material. This means that the material in the mouth part including the adjacent neck parts has properties which differ from those of the container body. In containers of, for example, PET, the material in the mouth part has a glass transition temperature Tg of 71.degree. C., whilst the glass transition temperature of the material in the container is about 81.degree. C. It follows from this that the material in the mouth part softens at a lower temperature than the material in the container body.
It is already known, by cold-forming of the mouth part of the blank, to displace material downwards from the mouth part into the sections of the blank, which later represent the wall sections of the container body. In this way, a certain matching of the quantity of material in the mouth part to the future stresses is achieved but, between the actual container body and the mouth part, neck sections are formed in which the material is stretched by a factor of less than 3. These neck sections in the moulded container thus consist of inadequately oriented material, whilst at the same time the wall thickness is undesirably large. This method is known from Swedish Patent Application No. 78/02,362-9.
French Patent Application No. 74/39,648 has disclosed a method wherein a tubular blank, which is closed at one end and which is provided at the other end with beading for fixing the blank in a downstream blowing element, is injection-moulded and wherein the tubular blank is blow-moulded after a certain reshaping to give a container. Material in the tubular part of the blank is expanded in the radial direction at a temperature above the glass transition temperature Tg in order thus to form the mouth part of the container. A container formed in the manner described possesses a mouth part and a neck section in which the material has been exposed to only very slight stretching and hence orienting, so that the disadvantages, already indicated, with respect to the mouth parts of the known containers are also present in this container. The invention described in the French application also has the disadvantage that only a part of the material content of the injection-moulded tubular blank is utilized when reshaping the blank to give the finished container. It is obvious that the losses of material, which occur in this process, represent an economic disadvantage in the mass production of articles.
From Federal German Offenlegungsschrift DOS No. 2,540,930, a process is known wherein a tubular blank of PET is reshaped to give a container and wherein the container wall consists of a material which is stretched by a factor of, for example, more than 1.5. The bottom part of the container consists of an amorphous unoriented material, whilst the neck sections of the container consist of material which has been oriented only to a slight extent. As a result of heating and crystallization, caused thereby, the strength of the material is improved in the unoriented zones which at the same time become opaque. Furthermore, a combination of the methods indicated above results in an undesired oversizing of the neck sections of the containers, whilst the latter at the same time have poorer properties than the material in the actual container body.